Conductive inks have been used for many years in a variety of applications. For instance, conductive inks have been printed on various substrate materials to form conductive circuits as are found in radio frequency identification tags, touch pads, and flexible circuits. Traditionally, conductive inks used for such applications included a dispersion of conductive particles, either metal (e.g., silver) or carbon, in a resin system designed to meet the specifications of both the formation process and the end-use application. Conductive inks have greatly reduced both the monetary and environmental costs associated with these circuits, as the traditional etching process for forming electronic circuits required many separate steps and generated significant hazardous waste problems. U.S. Pat. No. 5,189,952 to Ohmura, et al., U.S. Pat. No. 4,221,830 to Dalens, et al., U.S. Pat. No. 5,286,415 to Buckley, et al., U.S. Pat. No. 5,389,403 to Buckley, et al., and U.S. Pat. No. 5,656,081 to Isen, et al. exemplify several metal and/or carbon particle-containing conductive inks.
Recently, inherently conductive polymers have been examined as a possible replacement for the particles found in more traditional conductive inks. For instance, conductive polymer inks have been suggested for use in forming anti-static coatings, smart windows, corrosion control layers, EMI/RFI shielding, and photovoltaic applications. U.S. Pat. No. 6,358,437 to Jonas, et al. and European Patent Application Publications EP-A 1 081 549 and EP-A 1 081 548 exemplify some recent trends toward forming printable inks through addition of conductive polymers to the ink formulation.
Problems and room for improvement still exist in the art. For instance, traditional conductive inks, incorporating a dispersion of metallic or carbon particles, while often suitable for slower printing processes such as screen-printing, have been difficult to develop for higher speed processes, such as flexographic printing and rotogravure printing. In addition, these more traditional conductive inks often contain environmentally hazardous materials, e.g., volatile organic chemicals (VOC) as solvents such as xylene and toluene.
Similarly, conductive polymer inks have also proven difficult to prepare with suitable viscosities for high-speed printing processes such as flexo/gravure processes. This problem has primarily been attributed to the low solubility of the conductive polymers. In addition, once printed and dried, the conductive polymer inks have often failed to establish suitable conductivities for many applications, and in particular, for printed circuit applications.
What are needed in the art are conductive polymer inks that can be formulated with viscosities suitable for high-speed printing processes. Moreover, conductive polymer inks that utilize environmentally-friendly components, particularly solvents, would be of great benefit. In addition, the ability to print materials and provide a printed, dried ink that exhibits high conductivity would be of great benefit.